This invention relates to the field of spectrum analyzers, and more particularly to the field of differentiating between real and spurious responses from a spectrum analyzer.
Spectrum analyzers of the superheterodyne or swept-frequency type are based on the use of a mixer and a variable frequency local oscillator. The input signal to be analyzed is mixed with the output of a sweeping first local oscillator to produce a first intermediate frequency signal. Any incoming signal that mixes with a harmonic frequency of the sweeping first local oscillator produces an analyzer response. At any one time, however, the spectrum analyzer is calibrated for responses produced by mixing with only one of the harmonics of the sweeping first local oscillator. These responses are the "real" responses of the analyzer, while those produced by mixing with the other harmonics for which the analyzer is not presently calibrated are spurious.
For some types of spectrum analyzers within some frequency ranges, a fixed or tunable filter restricts the input to only the frequencies that produce real or calibrated responses. However, in other cases, it is necessary to do more to distinguish between the real and spurious responses.
Methods are known for distinguishing between the real and spurious responses of a spectrum analyzer. Typically, these methods rely on changing the setup of the spectrum analyzer's front end, so that real signals are shifted by a predicted amount, while spurious signals are shifted by some other amount. Creating these predictable shifts can be accomplished by changing the local oscillator frequencies and/or the bandpass of the filters. U.S. Pat. No. 4,568,878 describes the automation of such a process by a microprocessor, so that spurious signals are automatically eliminated from the display. Nonetheless, despite the availability of this method, some users prefer to identify real and spurious signals for themselves, not wishing to risk having the computer inadvertently discard data that they are interested in.
Another way to discriminate between real and spurious responses is to switch the first intermediate filter frequency and make a corresponding change to the second local oscillator frequency, such that the two changes cancel out for frequencies within the desired frequency range that are mixing with the desired harmonic of the first local oscillator, but do not cancel out for signals that are mixing with the wrong harmonic. This approach causes the real responses to maintain their original horizontal location when these changes are made, while spurious responses move to different horizontal locations.
A simplified version of a conventional superheterodyne spectrum analyzer is shown in FIG. 1. The input signal to be analyzed is received at an input terminal 2 and is mixed in a mixer 4 with the output signal of a sweeping local oscillator 6 at a frequency F.sub.lo1 determined by oscillator tuning circuit 27 and swept by ramp generator 26. The output signal of the mixer 4 is applied to two fixed-frequency first I.F. filters 8 or 10, to produce an output signal at one of two intermediate frequencies F.sub.if1a or F.sub.if1b. The output of 1st I.F. Filter-A 8 is applied to one port of mixer 12, where it is mixed with the output of second local oscillator A 15. The output of 1st I.F. Filter-B 10 is applied to one port of mixer 14, where it is mixed with the output of second local oscillator B 16. A switch 17 selects between the outputs of the two mixers 12,14.
The output of the selected mixer 12 or 14 is applied to a second intermediate frequency filter and amplifier stage, including a fixed-frequency variable resolution filter 18 and and a log amplifier 20. The output of the second intermediate filter stage 18,20 is detected by a detector 22 and applied to amplifiers and video processing circuitry (not shown) to a vertical information digitizing stage 24.
A ramp generator 26 generates a horizontal sweep ramp that is used to sweep the local oscillator 6 and also applied to a horizontal position information digitizer 28. The digital output of the horizontal position information digitizer 28 is representative of the instantaneous frequency of the local oscillator 10 and is used as address information by storage RAM 38 in storing vertical information, representing signal amplitude, derived from the vertical position information digitizer 24.
A second ramp generator 40 generates a digital ramp that drives the horizontal amplifier 32 and that also provides address information for reading out the vertical information stored in the storage RAM 38. The vertical amplitude information stored in the addressed memory locations of the storage stage 38 drives the vertical deflection amplifier 30 of the CRT 34. Consequently, the CRT 34 display represents variations in the power in the input signal (vertical axis) as a function of frequency (horizontal axis).
The overall operation of the analyzer is controlled by an internal computer 36 that has interactions with most of the parts of the analyzer, including those parts not shown in FIG. 1, to coordinate and control the functionality of the analyzer and to accept operator input through the user interface.
A response appears on the vertical axis of the analyzer display whenever the input signal includes a component at a frequency F.sub.s such that one of the following conversion equations is satisfied: ##EQU1##
N and M represent the number of various harmonics and can only assume integer values. And, with input signal levels low, N can be assumed to be always equal to 1. It is also necessary that the quantity in parenthesis be within the passband of the first IF filter stage 8. The first intermediate frequency is selected such that either the first three or the last three conversions are not possible. The first IF frequency is less than the minimum value of the first (sweeping) local oscillator frequency F.sub.lo1, so that conversions (3) and (6) are not possible. Thus, only an input signal F.sub.s that satisfies either of the following equations produces a signal on the display: EQU (M.times.F.sub.lo1 -F.sub.s)-F.sub.lo2 =F.sub.if2 ( 1a) EQU (F.sub.s -M .times.F.sub.lo1)-F.sub.lo2 =F.sub.if2 ( 2a)
However, the analyzer is calibrated, at any one time, for only one of these two equations and a particular value of M, say M.sub.l. Nevertheless, signals satisfying the other equation reach the display, where they are considered to be spurious because the frequencies indicated for them are always incorrect and the indicated amplitudes are also prone to error.
To check for spurious responses, the computer 36 can be programmed to change the position of switch 17 and the signals sent to the oscillator tuning circuit 27, thereby changing the frequency of the first local oscillator 6. One of the 1st I.F. filters 8,10 has a bandpass appropriate to one of the first local oscillator frequencies, while the other of the 1st I.F. filters has a bandpass appropriate to the other of the first local oscillator frequencies. Thus, there are two alternative paths through the spectrum analyzer front end that are both based on the same value of M (harmonic), but for different values of F.sub.if2. The difference in the two 1st I.F. frequencies is then cancelled out by a difference in the frequencies of the second local oscillators 15,16, so that either input to the variable resolution filter 18 as selected by switch 17 is in the same frequency range.
Switching between these two alternative paths through the spectrum analyzer front end causes real responses to appear in the same place on the display, while spurious responses show up in different locations or do not appear on the display at all.
In the prior art, it has been known to use a second path through the analyzer front end to generate a second set of sweep data, to store that alternative sweep data, and to display that data from the alternative setup vertically offset from the current data by a predetermined amount to permit an operator to make a visual comparison between the two.
However, this visual comparison is susceptible to error, and, if the analyzer controls are being operated so that the display is affected, the stored reference sweep data is no longer useful. What is desired is a better way for an operator of a spectrum analyzer to make easy and on-going comparisons between the signals traveling through two setups of the spectrum analyzer front end.